1. Field of the Invention
This invention relates to an electrode for a coulometric type of electrochemical detector capable of providing a highly sensitive detection of trace amounts of electrically active substances, a process for producing it and an analytical apparatus in which it is built.
2. Background Art
When used in combination with liquid chromatography, electrochemical detectors can detect, with very high sensitivity, trace amounts of electrochemically active substances contained in biopsy such as catecholamines and their metabolites. Thus, analytical apparatuses in which electrochemical detectors are combined with liquid chromatography have been used for the detection and quantitative determination of such electrically active substances.
An amperometric type of electrodes have so far been used as electrodes for electrochemical detectors, but they cannot be applied to more precise analysis because their efficiency of electrolysis is limited. Known as an electrode capable of carrying out detection with the highest sensitivity is one used for a coulometric type of electrochemical detector.
Essentially required for such an electrode for the coulometric type of electrochemical detector is that:
1) it be formed of a porous, electrically conductive material which has higher surface area than a working electrode for an amperometric type of electrolytic cell and which is of an open-pore structure, because it is required that the substances to be detected pass through an electrode cell while they are all electrochemically converted with good efficiency;
2) its surface possess a low polarity and be chemically stable since it is required to show durability during long-term use; and
3) the porous material be required to have a sufficiently large mean pore diameter and a constant pore size distribution so as to reduce as much as possible a pressure difference occurring in the electrode cell, because considerable limitation is imposed on analysis conditions when that pressure difference is in excess during solution passage.
Use of porous carbon materials having a high surface area has been studied as an electrically conductive material expected to meet these condition 1) to 3). For instance, a proposal has been made of an electrode for a coulometric type of electrochemical detector which is formed of a high-surface area, porous graphite, as set forth in U.S. Pat. No. 4,804,455.
However, a graphitic material has a defect of being so poor in chemical stability that it degrades by contact with organic solvents over an extended period. What is generally said of a porous material is that its surface area is in inversely proportional relation to its mean pore diameter or, in other words, the higher its surface area the smaller its mean pore diameter. Thus, in general, even when a graphitic material meets the above condition 1), it will fail to satisfy the condition 3), giving rise to an excessive pressure difference between electrodes and so making it unfit for practical analysis.
In the case of such a graphitic material having a high surface area, the pressure difference occurring during solution passage may be suppressed to some degree by increasing its surface area by forming a number of fine pores of the order of 1 .mu.m or less on the surface of each of particles constituting the material, while making the distance between particles larger, say 5 .mu.m or more. With this material, however, it has been found that residual currents are produced owing to penetration of a solution into the fine pores, making the S/N ratio worse. This defect is attributable to the fact that the graphitic material is not glassy or, in better words, the fact that the stack thickness of crystallite, Lc(002), as determined by X-ray diffractometry, is too large, say 80 .ANG. or more. Thus, it has turned out that it is also necessary for the surfaces of porous material forming particles to have a dense structure containing very few fine pores of 1 .mu.m or less and to be less impregnated with a solution.
Other graphitic materials have been proposed in French Patent No. 1,483,100 and U.S. Pat. No. 4,294,893. However, these materials have been found to have similar defects as referred to in connection with the above-mentioned U.S. Pat. No. 4,804,455.
As proposed in U.S. Pat. Nos. 4,506,028 and 4,814,307, there has also been available a material produced by calcining carbon fibers of 5 to 30 .mu.m in diameter. This material, which is neither graphitic nor glassy carbon, also has too large an Lc(002), i.e. 80 .ANG. or more, as determined by X-ray diffractometry, again making the S/N ratio worse. In addition, it cannot be used as an electrode over an extended period due to its poor chemical stability.
Moreover, Japanese Patent Laid-open Publication No. 54-41296 discloses a material obtained by calcining carbon black. This material is not a glassy carbon and also not suited for a long-term use as an electrode.
An object of this invention is to provide an electrode for a coulometric type of electrochemical detector designed to produce a pressure difference during solution passage therethrough, which, if any, is too small to place any limitation on the analysis conditions for liquid chromatography and keep the occurrence of residual currents low, thus enabling a highly sensitive detection with a high S/N ratio and exhibiting its improved electrochemical conversion characteristics over an extended period.
Another object of this invention is to provide an analytical apparatus including a column for liquid chromatography and the above-mentioned electrode for a coulometric type of electrochemical detector.